Color image sensors are used in various applications from simple and inexpensive mobile-phone cameras and digital cameras to sophisticated and expensive medical and scientific equipment. Conventional color image sensors include a pixel array having a color filter array (“CFA”) overlaying an array of photosensitive elements, which convert the intensity of the color filtered light into electrical signals.
The CFA typically includes three color filter elements for generating a three channel color output. Often three primary color filters are used to generate three primary color signals: red (R), green (G), and blue (B) signals. Thus, each pixel in the pixel array is sensitive to one of the three primary colors and outputs a corresponding color signal. For example, to produce a R signal, the pixel is covered by a red color filter element, to produce a G signal, the pixel is covered by a green color filter element, and to produce a B signal, the pixel is covered by a blue color filter element. Accordingly, at least three different color filter elements are used. The three different color filter elements use three separate fabrication procedures to fabricate the complete three color element CFA.
FIG. 1 illustrates an example Bayer pattern CFA 10 formed using R pixels, G pixels, and B pixels. CFA 10 generates a three channel color output of R, G, and B signals. CFA 10 may have hundreds or even thousands of rows and columns. The Bayer pattern CFA uses a repeating pattern of macropixel groups 20. Each macropixel group 20 includes four micropixels 30: a R pixel, a B pixel, and two G pixels. A single macropixel group 20 produces a R signal, a G signal (the two G signals are combined), and a B signal. Each macropixel group 20 may be considered the smallest element of a color image produced by the color image sensor, where each macropixel group 20 corresponds to a single image pixel in the output image data. In other words, the resolution of the output image directly corresponds to the linear density of macropixel groups 20. The R, G, and B signals output by the individual micropixels 30 of a single macropixel group 20 are combined to form an individual color image pixel in the output color image data.
The extraction of R, G, and B signals from each macropixel group 20 is straightforward. The R pixel produces the R signal, the G pixels produce the G signal, and the B pixel produces the B signal. Of course, the G signal is larger than either R or B signal for the same light intensity, which mimics the human eye's higher sensitivity to green light compared to red and blue light. Thus, conventional color image sensors have a one-to-one correspondence between the number of distinct colors in the filter elements of the CFA and the number of output color channels.